WO 02/069791 describes a device for measuring blood glucose in living tissue. It comprises an electrode arrangement with a ground electrode and a signal electrode. A signal source applies an electrical AC-signal of known voltage or current through a resistor to the electrodes, and a detector determines the voltage over the electrodes. This depends on the dielectric properties of the tissue, which, as it has been found, are indicative of the glucose level within the tissue.
Devices of this type have been found to deliver good results, but they require a substantial effort for calibration and are prone to drift, in particular when the environmental conditions change or when they are being displaced. Differential methods have been used to improve measurement accuracy in other fields of technology. Differential methods are generally based on two or more measurements, wherein one or more measurement conditions are changed between measurements. Depending on the specific application, the influence of some parameters can be eliminated or reduced by calculating the difference between the measurements.